reserve SAS for Semi_Affine_Space;
reserve a,a9,a1,a2,a3,a4,b,b9,c,c9,d,d9,d1,d2,o,p,p1,p2,q,r,r1,r2,s,x, y,t,z
  for Element of SAS;

theorem Th5:
  a,b // c,d implies a,b // d,c
proof
  assume a,b // c,d;
  then c,d // a,b by Th2;
  then d,c // a,b by Th4;
  hence thesis by Th2;
end;
